At a time of posting on a website or acquiring information from a website, in order to prevent a fraudulent use or an attack by automatic processing performed by using a computer, it is sometimes verified on a server side whether or not a subject operating a computer is a human. For example, Patent Literature 1 below discloses a technology for displaying a character string image (CAPTCHA) which is easy for a human to decipher but is difficult for a machine to decipher, and prompting a user to input the displayed character string so as to determine whether or not the subject operating the computer is a human based on whether the input character string is right or wrong.